Merging flocks

I have 5 RIR's that are about 1 1/2 yrs old and 6 Golden Buffs that are abour 6-7 months old in another enclosure. The Buffs have been laying for about 2 weeks and I need to move them into the coop with the Reds. I am afraid that there will be trouble.
The Buffs never seem to fight or squabble, but the Reds are a different story. Any suggestions on how to intergrate the flocks together?

Can you flip flop them? Put the new girls in the coop and the others in the moveable......then add the older girls back into the coop after some time (not sure how long, maybe others can weigh in). That may give the young ones a better chance?

My best shot at integrating has been doing it while free ranging. Sometimes they have to run 50 feet to get away from the chaser....free ranging they can do that and never even get pecked. In the coop they always get run down and jumped on.

So, just got rid of a rooster and the big girls (10 weeks) are working out the pecking order. They are a month older than the little girls, which include two bantams. How long should I wait until I try to merge the two flocks? they've been free ranging separately for a couple of weeks, but big girls still try to menace the the little girls through the fence. the rooster actually had the Houdini(because she's always getting out) bantam pinned under his big ol' foot while trying to rip off her comb, that's why he went to live in the country until it's time for freezer camp. Any thoughts or ideas on the merging? I've heard at night, in the coop. I've heard while they free range. I've heard keep the older ones in the run and the new ones in the coop, but then they don't interact at all. I just dont' want blood and feathers everywhere. Are my bantams okay with the others? They're fine with the little girls, but will soon be outgrown by them. just trying to get people to weigh in on what they've tried and what has worked and just as important, not worked.